In geodetic instruments, such as transits and theodolites comprising a stationary base, an alidade standard which rotates upon the base about a vertical spindle, and a telescope supported by the standard to rotate about a horizontal axle, means are normally provided which will enable precise, minute displacement of the alidade standard and the telescope about their respective axes during the acquisition of a target under observation. Such adjustment systems usually comprise means for clamping an extending arm member to the axle or spindle, and means, usually including an advancement screw member, for displacing the distal arm end to achieve the desired adjustment. This type of "tangent screw" system particularly adapted for use in a transit surveying instrument is described in U.S. Pat. No. 3,093,904.
In such earlier adjustment systems the respective locking means and screw advancement means were positioned about the instrument in relatively widely separated locations; however, such arrangements were not then inconvenient, since the operator was required by the external scale arrangements to move about the instrument when taking readings and could thus not normally maintain a casual stationary position at the eyepiece of the telescope. With the advent, however, of the electronic geodetic instrument, such as that described in U.S. Pat. No. 4,146,927, having digital readout means located in the vicinity of the telescope eyepiece, the operator is able to maintain a working position behind the instrument and as a result has come to demand that the displacement adjustment controls be located together at a near face of the instrument.
Those utilitarian demands were in part satisfied by the coaxial adjustment controls described in U.S. Pat. No. 4,084,327; however, their location at one side of the instrument resulted in a somewhat inconvenient displacement from right to left side during the plunging and reversing of the telescope. In the present invention, however, this shortcoming is eliminated in that matched control knob pairs by which each of the elevation and azimuth adjustment is effected are situated at both of the opposite working of the instrument, thereby enabling the operator to directly control adjustment regardless of the position of the telescope with respect to the alidade standard. Further, an additional advantage of this arrangement of control knobs in the faces of the instrument, rather than the side plate, is realized in that the adjustment system remains operative during maintenance procedures which normally require removal of the alidade side plate to obtain access to the instrument interior.